Electrodeposition of chromium

ABSTRACT

The invention provides an acidic aqueous solution for electrodeposition of chromium comprising chromic acid (CrO3), at least 2 grams of boric acid or a salt giving rise to borate ions per liter of solution, and sulphate ions.

Q United States Patent 11 1 1111 3,713,999 Law et al. 1 1 Jan. 30, 1973[54] ELECTRODEPOSITION OF [56] References Cited CHROMIUM UNITED STATESPATENTS [75] Inventors: Malcolm John Law, Dorking; 3 303 H4 2/967 Rom k,204/51 1 Y anows 1 55 g g Bmmsgmve 3,505,183 4 1970 Seyb etal ..204 51 00 3,514,380 5/1970 Tuchewriz .204/51 3,069,333 12/1962 Deyoup .204/51[73] Ass'gnee' Er (ihegmals 3,340,165 9/1967 Chessin ..204/51 3,418,22012/1968 R0ggendorf.... .204/41 [22] Filed: Oct. 12, 1970 PrimaryExaminer-F1 C. Edmundson [21] Appl. No.. 80,220 Att0rneyChristen & Sabol1301 Foreign Application Priority Data [571 ABSTRACT Oct. 10, 1969 GreatBritain ..49,802/69 The inventiofPmvides an i 9 electrodeposltlon ofchrom1um compnsmg chr0m1c cid (CrO at least 2 grams of boric acid or asalt 152 US. Cl .204/51 51 Int. Cl. ..C23b 5/06, C23b 5/48 if"? to mateper and p ate10ns. [58] Field of Search ..206/51 10 Claims, No DrawingsELECTRODEPOSITION OF CHROMIUM This invention relates to theelectrodeposition of chromium on nickel, cobalt or nickel-cobalt coatedmetal articles, e.g., chromium on nickel-coated steel articles andchromium or nickel-coated on to coppercoated zinc base alloy articles.

One object of the present invention is to produce a chromium coatingwhich has a large number of microcracks, uniform color of deposit, goodcathode efficiency and adequate throwing and covering power.

Numerous difficulties exist in providing an electrolyte solution capableof producing a bright microcracked chromium with good throwing andcovering power.

Numerous chemical compounds have been suggested for incorporation withthe electrolyte and they generally display some disadvantages.

Thus the addition of fluorides increase cathode efficiency but mayproduce non-uniform color deposit.

Sulphates are often added to chromium electrolytes and a wide range hasbeen suggested but not all proportions are suitable and suitableproportions vary with different electrolyte compositions.

Suggestions have been made to add selenium compounds to the electrolyte.

In known chromium plating baths containing a fluoride and sulphate amicrocracked deposit is obtainable with a high proportion of fluoride inareas of an article where current density is high but on the samearticle microcracking is not produced on areas of low current density. Ahigh fluoride content also will not tolerate the increasing oxidicimpurities which occur over a period of use and involves a criticalrange of sulphate which is impractical to maintain. A lower proportionof fluoride does not produce a good deposit. If selenium is included insufficiently large proportion with a view to facilitating the productionof microcracks, a deposit is produced which has an undesirable bluecolor and the covering power of the deposit over areas of varyingcurrent density is reduced. If the proportion of fluoride is reduced andthe solution is otherwise suitably adjusted, the deposit tends to havebands of different color across the article.

In addition to these problems, difficulties are experienced inmaintaining the solution over an extended period of use because thecomposition of the solution changes, e.g., an increasing proportion ofoxidic impurities. Thus the solutions heretofore proposed have thedisadvantage of needing continual readjustment.

The present invention provides a solution which gives a goodmicrocracked deposit of good covering power over areas of varying shapeand current density and good uniform color and which maintains thesecharacteristics over a long period with little or no maintenance, usinga low proportion of fluoride and an adequate range of sulphate.

Inclusions of boric acid in chromiufn plating baths has been consideredundesirable (e.g. page 179 of Electroplating Laboratory Manual" by R.C.Armet publishedby Robert Draper Limited of, Teddington which states thatpoor rinsing from a bright nickel solution will result in nickel, boricacid, sulphate and chloride entering the chromium bath, all troublesomeimpurities resulting in lowering of covering power"). Our own experienceconfirms that boric acid is commonly used in nickel plating baths andthis results in some boric acid being carried over with the chromiumplating baths with detrimental effects, e.g., boric acid added tocertain otherwise reasonably satisfactory electrolyte solutions causesloss of microcracks. As far as we are aware boric acid has never beenproposed as an addition to electrolytes for producing microcrackedchromium'coating.

The present invention is based on the discovery that the combination ofa high content of boric acid or of a salt giving rise to borate ions(B0,), in combination with selenium ions produces a surprisingcombination of advantages.

The boric acid or a salt giving rise to borate ions may be used inquantity from two grams per liter of electrolyte solution up to asaturated solution.

The incorporation of the boric acid or salt makes the electrolyte moretolerant to variation of the CrO /SO, ratios.

A suitable salt is sodium or potassium perborate or borax.

The electrolyte solution of the present invention gives a good brightcolor (less blue), is tolerant to a high proportion of oxides, has goodthrowing and covering power, good cathode efficiency, even distributionof microcracks, and tolerant to fluoride and wide range of sulphate.

For example, in accordance with the present invention, we may provide asolution for use as an electrolyte for electrolytic deposition ofchromium comprising an aqueous solution containing chromic acid (CrOboric acid or salt giving borate ions, a fluorine-containing compound, asulphate-ion-containing compound, and a selenium compound; the seleniumcompound containing from 0.003 to 0.012, e.g., 0.003 to 0.009 seleniumdioxide gram per liter, preferably 0.005 to 0.009; the quantity of CrObeing 180 to 250 grams per liter, and the ratio of CrO to fluoride ionis 110 to 320 to 1, preferably 190 to 220 to 1. The ratio of sulphateion to fluoride ion may be 0.64 to 2.56 to l for best microcrackeddeposit best covering and throwing power. The ratio CrO to S0, may be to250, for best microcracked coating.

The boric acid may be used in any proportion up to maximum solubilitybut we normally prefer 10-30 grams per liter.

The average operating temperature may be to F, preferably l05-115 F. Thecathode current density may be -200 amps per square foot. The period oftreatment is normally 6 to 10 minutes.

We have found that, in the absence of boric acid or borate salt, morethan. .006 g/l selenium dioxide will cause the cracks to become too fineor may be lost al' together.

A plating time of a minimum 7 minutes at 200 amps/sq.ft. will producemicrocracked deposits having cracks of some 700-2000 per linear inch.Deposits of less than 0.00002 inch Cr. surprisingly showed a microporousstructure of fine pit-like pores and small cracks that failed to join upas those at greater thickness.

This type of structure was also evident at extremely low currentdensity. A Hull Cell panel plated at 8 amps in this electrolyte for 7minutes showed a deposit having a crack pattern extending for the first2% inches from the high current density end and a porous patternextending to 3% inches from the high current density end.

This showed a covering power, i.e., 3% inches of chrome deposit at 8amps. In a l-lull Cell this could not be obtained from other similarprocesses, of uniform color and micro-discontinuities over the entireplating range.

Thus articles of complex shape can be plated by this process and even inlow current density areas and/or low thicknesses of chromium improvedcorrosion resistance can be found by virtue ofhaving microcracks.

The invention also provides a combination of additional advantages, viz.wide current density range, wide temperature range, low fluorideconcentration, microcracks over the entire plating range at allthicknesses, and ability to operate satisfactorily with fluorinatedhydrocarbons in order to suppress chromium fumes and spray.

A particular example of an electrolyte solution in accordance with thisinvention consists of:

.007 g/l. selenium oxide 225 g/l. chromic acid 1.5 g/l. potassiumsilico-fluoride 1.7 g/l. sulphuric acid 20 g/l. boric acid Temperature115 F.

Cathode current density 150-200 A.S.F.

Thickness of deposit .00003 inch.

In general the electrolyte solution of the present invention can be madeup as follows:

Chromic acid 175-450 g/liter.

Sulphuric acid 0.7 to 2.0, e.g., 0.7-1.0 g/liter.

Potassium silicofluoride 1.5 to 3 (fluoride ion 0.78

to 1.2), e.g., 1.5-2.5 g/liter (fluoride ion 0.78 to 1.04).

Selenium compound 0.003-0.009 g/liter.

Boric acid 10 to 30, e.g., -20 g/liter.

Other desirable ratios are as follows:

CrO to S0, 85 to 250 preferably 135-185 for best microcracking. CrO toS0, ion plus fluoride ion 45 to 140. Sulphate plus fluoride ion to Seion is 212 to 1780. Sulphate to selenium ion is 120 to 1000. Fluorideion to selenium ion is 90 to 320.

The number of microcracks per inch in the final chromium deposit is750-2500, preferably 1500-2000.

The fluorine compound may be hydrofluoric acid and/or its salts;fluoboric acid and/or its salts; fluozirconate, fluoaluminate,fluotitanate, sodium and/or potassium silicofluoride.

The selenium may be added as selenic acid, or its alkali metal salts, orselenious oxide.

The solution may also include other agents such as surface active agentsand foam depressants.

The thickness of the microcracked chromium layer may in practice be from0.00002 to 0.00004 inch.

We claim:

1. An acidic aqueous solution for electrodeposition of chromium inmicrocracked condition having at least v 700 microcracks per linear inchcomprising (a) chromic acid (CrO (b) at least two grams per liter ofsolution ofa boron containing compound selected from the groupconsisting of boric acid and water soluble salts thereof giving rise toborate ions, 50) sul hate ions, (d) a fluorine containing compoun and e)a selenium compound. I

2. A solution as claimed in claim 1, containing at least ten grams ofsaid boron containing compound per liter of solution.

3. A solution as claimed in claim 1 wherein said selenium compound isselenium dioxide present in an amount of at least 0.003 grams/litersolution, the quantity of CrO is 180 to 450 grams per liter and theweight ratio of CrO to fluoride ion is :1 to 266:1.

4. A solution as claimed in claim 3, wherein the ratio of sulphate ionto fluoride ion is 0.64:1 to 2.56:1 and the ratio ofCrO to S0, is 100:1to 180:1.

5. A solution as claimed in claim 3, wherein the ratio of fluoride ionto sulphate ion is 2.26:1 to 2.56:1 and the ratio of CrO to S0 is 100:1to 180: 1.

6. A solution as claimed in claim 3, wherein the solution contains 0.005to 0.009 grams per leter of selenium dioxide in the selenium compound.

7. A solution as claimed in claim 1 wherein the proportions of theingredients are:

Chromic acid 175-450 g/liter Sulphuric acid 0.7 to 4.5 g/liter Potassiumsilico-fluoride 1.5 to 3 (fluoride ion 0.78 to 1.2) g/liter.

Selenium compound at least 0.003 g/liter selenium oxide Boric acid atleast 10 g/liter.

8. A process of electrodepositing chromium having 700 to 2000microcracks per linear inch on nickel, cobalt or nickel-cobalt coatedmetal articles, wherein the articles are treated in an aqueous solutionas defined in claim 1 at an average cathode current density of 150 to200 amps per square foot and a temperature of 100 to F for 6 to 10minutes.

9. An article having a microcracked chromium coating prepared accordingto the process of claim 8.

10. A process of preparing article having a microcracked chromiumcoating having 700 to 2000 microcracks per linear inch which compriseselec trolytically treating as the cathode an article selected from thegroup consisting of nickel articles, cobalt articles, nickel-cobaltarticles and articles coated with nickel, cobalt or nickel-cobalt in anacidic aqueous solution consisting essentially of (a) 180 to 400 g/literCrO at least 10 g/liter of a boron containing compound selected from thegroup consisting of boric acid or salts thereof, sulfate ions present ina weight ratio of CrO to S0,, 100:1 to 180:1, a fluorine containingcompound selected from the group consisting of hydrofluoric acid,fluorboric acid, salts of fluorboric acid, fluozirconate, fluoaluminate,fluotitanate, sodium silicafluoride and potassium silicafluoride in anamount providing fluoride to CrO in a weight ratio of 1:1 10 to 1:320and at least 0.006 g/liter, calculated as selenium dioxide, of aselenium compound selected from the group consisting of selenic acid,alkali metal salts thereof and selenious oxide, for about 6 to 10minutes at 100 to 120 F with an average current density of -200 amp/ft.

I i 1 i i

1. An acidic aqueous solution for electrodeposition of chromium inmicrocracked condition having at least 700 microcracks per linear inchcomprising (a) chromic acid (CrO3), (b) at least two grams per liter ofsolution of a boron containing compound selected from the groupconsisting of boric acid and water soluble salts thereof giving rise toborate ions, (c) sulphate ions, (d) a fluorine containing compound, and(e) a selenium compound.
 2. A solution as claimed in claim 1, containingat least ten grams of said boron containing compound per liter ofsolution.
 3. A solution as claimed in claim 1 wherein said seleniumcompound is selenium dioxide present in an amount of at least 0.003grams/liter solution, the quantity of CrO3 is 180 to 450 grams per literand the weight ratio of CrO3 to fluoride ion is 110:1 to 266:1.
 4. Asolution as claimed in claim 3, wherein the ratio of sulphate ion tofluoride ion is 0.64:1 to 2.56:1 and the ratio of CrO3 to SO4 is 100:1to 180:1.
 5. A solution as claimed in claim 3, wherein the ratio offluoride ion to sulphate ion is 2.26:1 to 2.56:1 and the ratio of CrO3to SO4 is 100:1 to 180:1.
 6. A solution as claimed in claim 3, whereinthe solution contains 0.005 to 0.009 grams per leter of selenium dioxidein the selenium compound.
 7. A solutIon as claimed in claim 1 whereinthe proportions of the ingredients are: Chromic acid - 175-450 g/literSulphuric acid - 0.7 to 4.5 g/liter Potassium silico-fluoride - 1.5 to 3(fluoride ion 0.78 to 1.2) g/liter. Selenium compound - at least 0.003g/liter selenium oxide Boric acid - at least 10 g/liter.
 8. A process ofelectrodepositing chromium having 700 to 2000 microcracks per linearinch on nickel, cobalt or nickel-cobalt coated metal articles, whereinthe articles are treated in an aqueous solution as defined in claim 1 atan average cathode current density of 150 to 200 amps per square footand a temperature of 100* to 120* F for 6 to 10 minutes.
 9. An articlehaving a microcracked chromium coating prepared according to the processof claim 8.